Electrically conductive covering material

ABSTRACT

A covering material comprising a first layer, which forms the wearing layer of the structure and contains an ionomer, as well as a second layer next to the first layer. According to the invention, the first layer contains a wear-resistant and electrically conductive ionomer polyelectrolyte and the second layer is electrically conductive. The first layer comprises in particularan ionomer, which is a polymer of a co- or terpolymer of ethylene and acrylic or methacrylic acid, or any other known ionomer, and a block polyether, the polymer being ionically crosslinked with alkali and/or alkaline-earth and/or metal cations. The second layer forms the bottom layer of the structure, and it is made up of a polyolefin elastomer that has been rendered electrically conductive. The material according to the invention is suitable as covering for ESD applications.

The present invention relates to a covering material comprising a firstlayer, which forms the wearing layer of the structure and contains anionomer, as well as a second layer next to the first layer. According tothe invention, the first layer contains a wear-resistant andelectrically conductive ionomer polyelectrolyte and the second layer iselectrically conductive. The first layer comprises in particular anionomer, which is a polymer of a co- or terpolymer of ethylene andacrylic or methacrylic acid, or any other known ionomer, and a blockpolyether, the polymer being ionically crosslinked with alkali and/oralkaline-earth and/or metal cations. The second layer forms the bottomlayer of the structure, and it is made up of a polyolefin elastomer thathas been rendered electrically conductive.

A material such as this usually comprises a first layer which forms thewear layer of the structure and which contains an ionomer, as well asecond layer next to the first layer.

By ‘covering material’ is meant in the present invention a structurewhich is at least in part made from a polymer and which can be used forcovering surfaces. By means of covering, a surface can be protectedmechanically and chemically, and desirable novel or improved propertiescan be obtained for the surface, such as the electrical conductivity ofa desired level in the present context. The covering material can alsobe used for decorating a surface. Surfaces to be covered include floorsand walls and ceilings, as well as various objects and parts, such asshelves and shelf tops, as well as desk pads and worktops.

In many applications the covering material must be electricallyconductive so that accumulation of static electricity can be prevented.Likewise it is important that sparking and other drawbacks caused bystatic electricity can be avoided in certain areas. For example, inoperating theaters in hospitals electrically conductive floor coveringsare used because of readily inflammable anesthetic gases and sensitiveinstruments.

Polymeric covering materials can, by known techniques, be renderedelectrically conductive, for example by mixing conductive particles withpolymers, as is done, for example, in U.S. Pat. No. 5,516,546, whereinelectrical conductivity is achieved using amorphous graphite powder orcarbon fiber or a mixture of these. Several conductive particles, suchas carbon black, become extracted from plastics, and their use in cleanrooms is problematic. Carbon black and graphite powder have beengenerally used for rendering polymers electrically conductive. Thecompounds thus produced are black or very dark in color. In coveringmaterial applications this is a factor that strongly limits the outerappearance. It should be noted further that electrical conductivityachieved by means of conductive particles is, owing to percolation,often either too high or too low.

It is also a conventional method to add to surfacings antistaticmaterials, which typically absorb moisture from the air, and thus formslight electrical conductivity. Antistatic materials gradually migrateto the product surface, wherefrom they tend to be rinsed off inconnection with, for example, washing. Consequently, the conductivity ofthe covering changes as the concentration of the antistatic materialdecreases. Electrical conductivity produced by means of antistaticmaterials varies strongly according to air humidity. This is anespecially adverse property, since in winter, when air humidity is low,the coverings do not conduct electricity sufficiently. The optimal ESDshielding range in terms of electrical conductivity is a surfaceresistivity range of 10⁶–10⁹ ohm. This range is difficult to achieve bymeans of antistatic compounds.

Various covering materials have also been described in the patentliterature. Thus, U.S. Pat. No. 6,282,848 discloses a floor and wallsurfacing material which is intended for a magnetically isolated roomand which is a combination of an electrically conductive and anelectrically non-conductive material which have been placed in layersand in which the conductive layer is nickel.

From U.S. Pat. No. 5,988,460 there is known an electrically conductivesurfacing wherein ferrosilicate particles are dispersed as particles ofapprox. 20 μm in a polymer resin.

U.S. Pat. No. 5,631,311 relates to a transparent material dischargingstatic electricity, wherein the electrical conductivity is producedusing a tin oxide powder having a particle size so small that it doesnot hamper transparency.

U.S. Pat. No. 5,307,233 describes how small flakes can be prepared frompolyvinyl chloride by continuous extrusion and by cutting these parts,coated with an electrically conductive covering, into bits. The bits aresubsequently mixed with a PVC material, and the mix thus obtained isused for manufacturing, for example, floor coverings.

PVC is at present a non-recommended material, and therefore it isdesirable to avoid its use in all surfacings. In case of fire, PVCreleases chlorine, which forms with water hydrochloric acid, which ishighly detrimental owing to its corrosive action.

U.S. Pat. No. 5,728,476 (Amtico Company Limited) discloses how from anionomer and a polyfunctional olefin compound a clear surface layerhighly resistant to wear is obtained for a resilient floor covering. Theionomer is crosslinked by means of the polyfunctional compound. Thisstructure in itself is not electrically conductive.

U.S. Pat. No. 4,083,824 (Armstrong Cork Company) describes how from atleast two alkylacrylate polymers and an ionomer a covering free ofpolyvinyl chloride is obtained by using mineral fillers. This structurein itself is not electrically conductive.

U.S. Pat. No. 5,616,418 (Atochem) describes a multilayer film thatcontains a block polyether amide. The structure according to the knownoption is not, however, electrically conductive. The publicationcontains no mention of the effect of the metal salts used as optionalcomponents on the electrical conductivity of the material.

U.S. Pat. No. 5,652,326 (Sanyo Chemical Industries, Ltd.) describes howan electrically conductive plastic is obtained from polyetherester amideand an alkali metal when 0.01–2.00 molar per cent of an alkali metalhalide or an alkaline-earth metal halide is introduced into the mixture.No differentiation is made between monovalent and bivalent ions withrespect to electrical conductivity. According to the known option,sulfone groups grafted into it bind the alkali cations. Carboxylic acidgroups are discussed in the description, but in the examples they arealways esterified. According to the examples, the recommended amount ofmetal salt is up to 5–30 molar per cent of the material being produced.Halogens cause problems in some practical applications.

U.S. Pat. No. 5,059,474 (Nitto Boseki Co.) relates to a multilayer floorcovering wherein the surface layer is prepared from softened polyvinylchloride. The structure is not electrically conductive.

JP application publication 58015554 (Toray Industries) discloses a blendof a strong polyetherester amide and an ionomer, resistant to heat andcold. The metal ion in the ionomer is mentioned as being(I)–(III)-valent. There is no mention of the electrical conductivity ofthe blend, and no distinction is made among different metal ions.

The object of the present invention is to eliminate the disadvantagesassociated with the prior known options and to provide a coveringmaterial of an entirely novel type.

The present invention is based on the idea that the covering structureis made of two adjacent layers, of which the first—in the case of afloor covering the “upper one”—is formed from an electrically conductiveionomeric polyelectolyte (IPE) and possibly fillers. The electricallyconductive ionomeric polyelectrolyte is of the type described in patentapplication PCT/FI02/00559, the contents of which are incorporated intothe present application by reference. This ionic polymer is in generalcharacterized in that it is made up of at least two separate polymers,one of them binding monovalent cations with the help of carboxylic acidsin the polymer chain.

In addition to the first layer made up of IPE the structure has at leastone other layer, which has been rendered electrically conductive bymeans of, for example, carbon black, graphite or carbon fiber. The firstlayer and the second layer are in electrical contact with each other,which means that they may be directly one against the other, or they areinterconnected by means of a material layer that does not act as aninsulator.

More precisely, the option according to the invention is characterizedin that the first layer contains a wear-resistant and electricallyconductive ionomer polyelectrolyte, and the second layer is electricallyconductive.

The invention provides considerable advantages. Thus, by means of theinvention there is obtained an electrically conductive, dyable andwear-resistant structure, in particular a covering structure such asfloor covering or wall covering, tile or mat or the like. Air humidityand temperature do not have major effect on the electrical conductivityof the covering material, as they do in the case of materialscontaining, for example, antistatic compounds. The color of the coveringaccording to the invention can be selected freely. It is preferably alsodevoid of halogens, which means that it will not produce toxic andharmful gases in case of fire.

The other advantages and characteristics of the invention are evidentfrom the following detailed description.

As was noted above, the covering material according to the inventioncontains at least two layers, of which the first layer contains as theelectrically conductive component an ionomeric polymer made up of atleast two different polymers, of which at least one binds monovalentcations by means of carboxylic acid groups grafted into the polymerchain. The second layer is an electrically conductive layer with whichthe first layer is in electrical contact.

Typically the first layer discharges the voltage, for which reason itsresistance is in the order of 1–100 Mohm (surface-surface, 100 V). Theresistance of the second, electrically conductive layer for its part isapprox. two decades smaller, i.e. approx. 0.01–0.2 Mohm. The surfaceresistance of the sheet is ≦100 Mohm (preferably less than 100 Mohm)measured according to Standard ASTM D-257.

The electrically conductive polymer component in the first layer is inone embodiment made up of at least two different polymers, of which onecontains carboxylic acid groups and the other ether bonds, and of atleast one alkali metal cation. According to the invention at least someof the ether groups hold by means of a polar charge a monovalent cation,which is Li, Na, K, Cs or Rb, or a mixture thereof. The cation is highlypreferably K. This and other cations (also the alkaline-earth ionsmentioned below) and similar compounds of bivalent cations can beintroduced into the blend as hydroxides, oxides, formates, carbonates,acetates or mixtures thereof. In the polymer blend, some of thecarboxylic acid groups are also ionized.

Among, for example, the copolymer of ethylene and methacrylic acid(E/MAA) and the polyether block amide (PEBA) and the alkali metal cationit is possible to build a polymeric system wherein an IPN(InterPenetrated Network) structure of PEBA is formed inside the E/MAAphase. In the material, some of the cations crosslink the methacrylicacid groups in the E/MAA. Thus there are formed thermally reversible ionbonds that improve the mechanical properties of the polymer. Some of thecations bond to the oxygen pools of the polyether and produce, forexample by means of segmental movement of the polymer chains, ionicelectrical conductivity. Instead of PEBA it is possible to use, forexample, a block polymer of polyester and polyether, or a block polymerof polyurethane and polyether. The polyether is such that it is capableof solvating ions.

The ionomer can be called polymer component A and the block polyetherpolymer B. Polymers A and B are present in the mixture at weight ratiosA/B 90/10–10/90, preferably 85/15–20/80.

Ionomers are known for, for example, their brightness and goodmechanical properties. Generally ionomers are copolymers of alpha- orbeta-unsaturated carboxylic acid and ethylene and are partly crosslinkedwith I-or II-valent cations. Ethylene ionomers are typically goodinsulators, and their surface resistances are of the order of 10¹⁶–10¹⁸ohm (10exp16–10exp18).

According to the invention, the ionomer component of the polymer blendcan be prepared, for example, from copolymers or terpolymers of ethyleneand alpha- or beta-unsaturated carboxylic acids, the copolymers orterpolymers containing, in addition to the above-mentioned mers, estersof alpha- or beta-unsaturated carboxylic acids. The carboxylic acid ingeneral has 3–8 carbon atoms. Typically the polymer has, in parts bymass, acrylic or methacrylic acid 4–24 parts, methyl-, ethyl, orbutylacrylate or vinyl acetate 0–40 parts, the balance being ethylene in100 parts of the polymer. Commercially available co- and terpolymersaccording to the invention include Basel's Lucalen, Du Pont's Nucrel,Bynel and Surlyn or Exxon Chemicals's Iotek-named ionomers and theirnon-neutralized precursors.

The polyether block may be located in the copolymer of the polyamide orpolyester or polyurethane. The polyether block may be composed ofpolyethylene or polypropylene glycols (polyethylene oxide orpolypropylene oxide), copolymers/blend polymers of these, poly(1,2-butylglycol), or poly(tetramethyl glycol). Typically the mass proportion ofpolyether in the copolymer is 20–90 parts of 100 parts. Most preferablyit is 50–90 parts of 100 parts. A low ether concentration weakenselectrical conductivity. Commercially available polymers that contain apolyether block include Hytrel (Du Pont) and Pebax (Atofina). Thesurface resistances of these polymers range from 3⁸ to 4¹³ ohm.

An example of a preferable polyether block of polymer B is polyethyleneoxide having a molecular weight within the range 300–20,000.

The cation content in the polymer blend is 0.04–2.5 millimols/gram ofthe polymer blend and the alkali cation content is 0.4–1.7millimols/gram of the polymer blend.

The ionomer in the surface layer may in general contain 0.1–3% by weightof an alkali metal cation and 0–2% by weight of an alkaline-earth metalcation, and 0–1% by weight of one or more of the following: zinc,magnesium and aluminum. The cations may be introduced into the ionomerin the form of hydroxide, silicate, formate, acetate, carbonate oroxide, e.g. potassium hydroxide or potassium silicate. Also other cationsources can be used.

The good mechanical properties of ionomers enable the material to beused in the wear layers of the floor coverings and similar coveringmaterials according to the present invention. Ionomers are very tough,and any scratches on their surface can be repaired simply by using ahot-air blower or, for example, a hot flat iron and siliconized paper. Acomparison with PVC plastic shows that in the course of time theproperties of PVC plastic become, primarily owing to the evaporation ofsofteners, brittle and hard. In ionomers, instead, as a consequence oftheir long useful life, the properties remain unchanged for a long time,which is especially important in covering material applications.

The first layer may be substantially completely transparent andelectrically conductive.

The polymer used in the electrically conductive second layer, i.e. thebottom or support layer, is typically a halogen-free polymer, such as apolyolefin elastomer. Even other elastomers are usable. Polypropyleneelastomers, polyethylene elastomers, ethylene-methacrylic acid butylacrylate terpolymers and polyurethane elastomers can be mentioned asgeneric examples. Commercial products that can be mentioned includeMontell's Adflex Q100F and Lucobit's Lucopren 1721 TPE/PP polymer.Electrically conductive particles are used typically in an amount of10–200%, preferably approx. 15–80% by weight, of the mass of the polymerphase. These fillers may be, for example, carbon black, groundanthracite, ground graphite, carbon fibers, and mixtures thereof.Various metal powders also work well.

The first and second layers may be in direct contact with each other, inwhich case the structure can be prepared by producing the polymer filmsby means of, for example, extrusion or a roll mixer, and by laminatingthe layers together by using heat and pressure. The extrusion of thefirst layer can be carried out at a temperature of 110–330° C. Theionomer is typically an adhesion polymer, and good lamination isachieved at relatively low temperatures, such as 110–250° C. Thepressures depend on the temperature and the time, but at a pressure aslow as 10 bar, generally approx. 10–25 bar, it is possible to produce alaminate according to the invention.

The thickness of the first layer is in general approx. 0.05–2 mm,preferably approx. 0.08–1 mm, especially preferably approx. 0.1–0.8 mm.The thickness of the second layer is in general approx. 0.08–3 mm,preferably approx. 0.1–2 mm, especially preferably approx. 0.2–1.5 mm.The thickness of the first layer and the second layer together isapprox. 0.5–4 mm.

It is possible to add to the surface layer, for example, quartz orfeldspar powder to increase resistance to wear. It is also possible touse other fillers (finely divided silica, aluminum oxide). The IPE usedin the structure according to the invention seems to bind differentfillers to the polymer matrix substantially better than, for example,PVC is capable of binding. For this reason a covering such as this issubstantially more resistant to wear than are conventional coveringmaterials.

It is possible to blend in the surface layer some other polymer, such asa polyolefin (polypropylene, polyethylene) to modify the properties ofthe surface layer and to reduce the raw material costs. Thecompatibility window of IPE is, however, quite wide, and in addition tothe said polymers it is also possible to blend with the ionomericpolymer, for example, polyamide, polystyrene or polyester in order toincrease stiffness and/or hardness.

To strengthen the bond between the filler and the polymer matrix, thefiller can be preferably treated with tetraethoxysilicate, whichgradually hydrolyzes to silica gel under the effect of moisture andbinds the polymer and quartz sand well together. This hydrolyzation israpid especially when the mix contains alkalinity as does IPE.

The described structure advantageously allows fluorescent chemicalsand/or pigments as well as phosphorescent pigments, the luminescence ofwhich remains for minutes after lights are turned off, to be added tothe surface layer. Such a covering can be used for marking emergencyexits.

As the surface layer of the electrically conductive covering accordingto the present invention is clearly anionic in character, it canespecially advantageously be waxed with a cationic wax, and thereby goodadhesion is attained.

According to one preferred embodiment, the electrically conductive andwear-resistant structure, especially a covering structure such as afloor tile or mat or the like, which is made up of at least two layers,contains a surface layer made up of an ionomer treated with awear-improving filler, the ionomer being a polymer of a co- orterpolymer of ethylene and acrylic or methacrylic acid, or any otherknown ionomer, and a block polyether, the polymer being ionicallycrosslinked with alkali and/or alkaline-earth and/or metal cations, inwhich ionomer the ratio of the ionomer to the filler is within the rangeor 10–90:90–10, and the bottom layer is made up of at least onehalogen-free elastomer. In this case, at least one filler in the bottomlayer is carbon black, ground anthracite or ground graphite.

The advantage of the structure according to the invention is that itselectrical conductivity can be regulated with precision, and thus it ispossible always to reach the optimal ESD shielding range, i.e. a surfaceresistivity range of 10⁶–10⁹ ohm.

Between the first layer and the second layer there may also be a thirdlayer, a so-called middle layer, which is made up of a polymer film thathas been rendered electrically conductive by means of pigments orpolymer blends known per se. If the ionomer layer is made transparent,it is possible to print patterns, texts, wood grain imitations andcolorings on the middle layer, according to need. The patterning orother design is in this case visible through the surface layer and thusgives the floor or other surface the desired outer appearance. Thethickness of the middle layer is in general 0.05–1 mm, preferablyapprox. 0.08–0.5 mm, preferably approx. 0.1 mm, and the concentration ofconductive particles is usually approx. 5–50% by weight of the layer,typically less than 30% by weight. The third layer is in general thinnerthan the first and second layers.

The three-layered structure can also be prepared by producing the layersseparately (in particular the first, transparent layer is produced byextrusion or roll mixer in the manner described above), whereafter theyare pressed together at a temperature of, for example, 150–180° C.,whereby there is produced a layer structure the electrical conductivityof which is after 14 days (at a relative humidity of 50% and atemperature of 23° C.) less than 30 Mohm according to Standard IEC61340-4-1.

According to another preferred embodiment of the invention there is thusproduced an electrically conductive multilayer structure, in particulara covering structure, such as tile- or mat-form floor covering, which ismade up of at least three layers, of which the top layer is transparentand is made up of an ionomer that is a polymer of a co- or terpolymer ofethylene and acrylic or methacrylic acid, or any other known ionomer,and a block polyether, the polymer being ionically crosslinked withalkali and/or alkaline-earth and/or metal cations. The middle layer ismade up of an electrically conductive layer of a polymer known per se,the layer having been rendered at least partly opaque with fillers orpigments. The bottom layer is made up of a polypropylene elastomer or acorresponding elastomer, rendered electrically conductive withcarbon-containing particles and/or fibers or other known electricallyconductive particles.

Between the first layer and the second layer there may be also a middlelayer of another kind, which is intended for dimensional stabilizationand is prepared from, for example, glass fibers or carbon fibers, or acombination thereof. The layer may be net-like. In uses in which themiddle layer may be grounded and the middle layer contains carbon fiber,the bottom layer may be, with respect to gluing, of any known materialbut not PVC, for the reasons described above. When the stabilizing layercontains carbon fiber in an amount making grounding possible, the lowerpolymer layer may, when so desired, be electrically non-conductive. If,on the other hand, the bottom layer is electrically conductive, thedimensional stabilization layer may be a glass fiber weave or a jointfabric of glass fiber and carbon fiber or a non-woven material, in whichcase the top layer and the bottom layer (first and second layers) can bepressed together through the net. The dimensionally stable material mayalso be of ceramic fiber or polyester or polyamide, or a mixturethereof, or a mixture of these or any of those mentioned above.

The middle layer stabilizes thermal and hygro-instability, and itsthickness is, for example, 0.1 mm. It may contain, for example, 15%carbon fiber.

An electrically conductive middle layer is preferable especially incases in which it is not desirable to add electrically conductiveparticles, such as carbon black or carbon, to the bottom layer. Theseapplications include floor coverings for clean rooms and desk pads.

According to a third preferred embodiment of the invention, theelectrically conductive and dimensionally stable covering structurecomprises two polymer layers, of which at least the top one iselectrically conductive and made up of an ionomer that is a polymer of aco- or terpolymer of ethylene and acrylic or methacrylic acid, or anyother known ionomer, and a block polyether, the polymer being ionicallycrosslinked with alkali and/or alkaline-earth and/or metal cations,there being between the polymer layers a material dimensionally stableagainst changes caused by moisture and temperature.

In this third material the layer called the “second” above is below thelayer called the “third”, and the said “third layer” is or may beelectrically conductive, in which case the “second” layer need not beso. It is essential in terms of the invention that the first,ionomer-containing layer is in electrical contact with at least oneother layer that is electrically conductive.

The structure according to the present invention is typically in theform of a tile or a mat. It can be used for purposes mentioned in thepreamble of the specification, in particular for covering floors, wallsand various flat surfaces. The invention is especially well usable forESD shield applications.

The invention is described below with the help of examples.

Acronyms Used

-   -   MAA, methacrylic acid    -   BA, butyl acrylate    -   E, ethylene    -   PEG, polyethylene glycol

EXAMPLE 1

A single-colored, white, elastic two-layer covering is prepared, whereinthe approximately 0.6 mm thick top layer consists of the followingcomponents:

Top Layer

-   -   70 parts of an E/BA/MAA terpolymer, in which MAA 10 molar % and        BA 20 molar %    -   30 parts of a block polymer of polyamide or polyester and PEG    -   200 parts of CaCO₃; particle size <100 μm    -   10 parts of titanium oxide    -   2.0 parts of KOH    -   0.2 parts of Zn acetate

The bottom layer, having a thickness of 1.3 mm, was prepared from thefollowing components:

Bottom Layer

-   -   60 parts of a polyolefin elastomer, Adflex Q100F    -   40 parts of an E/BA/MAA terpolymer    -   300 parts of calcium carbonate, ground    -   100 parts of carbon black    -   1.0 parts of magnesium oxide

Both of the layers were prepared in a roll mixer at a temperature of160° C., and were laminated together in a static press further at atemperature of 160° C.

Electrical conductivity was measured at 14 days from manufacture(humidity 30% RH), and it yielded a value of less than 20 Mohm,according to Standard IEC 61340-4-1. The surface resistance of the sheetwas <100 Mohm, measured according to Standard ASTM D-257.

According to the targeted use, the filling factors of the layers mayvary within the range 10–90 molar %.

EXAMPLE 2

A two-layer mat especially well suited for covering shelves wasprepared. The materials of both layers were prepared by extrusion, andsheets were pressed at 170° C. from the bands obtained and were furtherlaminated together at 120° C.

Top Layer

-   -   55 parts of E/MAA, MAA 9 molar %    -   15 parts of E/BA/MAA, MAA 10 molar % and BA 10 molar %    -   30 parts of polyether-ester amide, PEG 50 molar % and        polyamide-12 50 molar %    -   2.0 parts of TiO₂    -   2.2 parts of KOH    -   0.2 parts of MgO        Bottom Layer    -   100 parts of EVA, VA 16 molar %    -   20 parts of carbon black

The thickness of the top layer of the composite was 0.2 mm and that ofthe bottom layer was 0.8 mm. The surface-surface resistivity measuredfor the mat was 15 Mohm according to Standard IEC 61340-4-1 at 14 daysfrom manufacture, air humidity was 40 RH.

EXAMPLE 3

A two-layer tile especially well resistant to wear was prepared. Thematerials of both layers were prepared by extrusion, and from the bandsobtained there were pressed sheets, which were further laminatedtogether.

Top Layer

-   -   65 parts of E/BA/MAA, MAA 10 molar %, BA 10 molar %    -   35 parts of polyether-ester amide, PEG 50 molar % and        polyamide-12 50 molar %    -   250 parts of quartz sand, particle size less than 300 μm    -   10.0 parts of TiO₂    -   2.0 parts of KOH    -   0.4 parts of MgO        Bottom Layer    -   100 parts of EVA, VA 28 molar %    -   300 parts of CaCO₃ powder    -   80 parts of carbon black

The thickness of the top layer of the composite was 0.6 mm, and thethickness of the bottom layer was 0.6 mm. The surface-surfaceresistivity measured for the tile was 15 Mohm according to Standard IEC61340-4-1 at 14 days from manufacture, air humidity was 40 RH.

EXAMPLE 4

A two-layer mat having an especially attractive outer appearance andbeing suited for covering was prepared. The top layer had in itsembodiment excellent optical properties, and the electrical conductivityof the second layer was produced by means of conductive titanium oxide.In this case it is possible to print multi-colored patterns on thesurface of the white lower layer. The materials of both layers wereproduced by extrusion, and from the bands obtained there were pressedsheets, which were further laminated together.

Top Layer

-   -   65 parts of E/BA/MAA, MAA 10 molar % and BA 10 molar %    -   35 parts of polyetherester amide, PEG 50 molar % and polyamide        50 molar %    -   2.2 parts of KOH    -   0.2 parts of MgO        Bottom Layer    -   100 parts of EVA, VA 16 molar %    -   50 parts of electrically conductive titanium oxide

The thickness of the top layer of the composite was 0.4 mm and that ofthe bottom layer was 0.6 mm. The surface-surface resistivity measuredfor the mat was 11 Mohm at 14 days from manufacture, air humidity was 40RH.

The invention is described above with examples. It is, however, clearthat the material thicknesses are by way of example and may varydepending on the covering material used.

EXAMPLE 5

An elastic three-layer mat was prepared, wherein the 0.5 mm thick toplayer was composed of a dissipative plastic filled with feldspar, andthe 1.5 mm thick bottom layer of a polyolefin elastomer having groundcalcium carbonate as the filler. Between the layers there was anelectrically conductive glass fiber/carbon fiber net.

Top Layer

-   -   60 parts of E/BA/MAA, where BA=10 molar %, MAA=10 molar %    -   40 parts of polyether amide, where ether=PEG 40 molar %,        amide=copolyamide 6 and 6.6 60 molar %    -   1 part of MgO    -   1.5 parts of KOH    -   200 parts of feldspar; particle size <100 μm        Bottom Layer    -   100 parts of a TPE/PP polypropylene elastomer    -   60 parts of ground calcium carbonate

The top layer was prepared by extruding the components, except for thefeldspar, at 220° C. A strong increase in the melt viscosity of theplastic indicated the formation of ion bonds. The feldspar was mixedwith the prepared plastic by means of a roll mixer at 130° C.

The components of the bottom layer were mixed in an extruder at 220° C.The surface-surface resistance measured for the structure laminatedtogether was 30 Mohm at 14 days from manufacture, at a humidity of 40%RH.

1. A covering material comprising a first layer, which forms the wearinglayer of the structure and contains an ionomer, and a second layer, nextto the first layer, wherein the first layer contains said ionomer whichis a wear-resistant and an electrically conductive ionomerpolyelectrolyte, said ionomer is a polymer blend of a co- or terpolymerof ethylene and acrylic or methacrylic acid, and a block polyether, thepolymer blend being ionically crosslinked with alkali and/oralkaline-earth and/or metal cations, and the second layer iselectrically conductive.
 2. The covering material according to claim 1 ,wherein the second layer forms the bottom layer of the structure and ismade up of a polyolefin elastomer that has been rendered electricallyconductive.
 3. The covering material according to claim 2, wherein thesecond layer contains anthracite, graphite, carbon fiber, carbon blackor an electrically conductive polymer.
 4. The covering materialaccording to claim 1, wherein next to the first layer, in electricalcontact therewith, there is the second layer, which comprises a netprepared from an electrically conductive material, the net at the sametime reducing the thermal and hygro-instability of the electricallyconductive material.
 5. The covering material according to claim 1,wherein the first layer is at least substantially transparent and thatbetween the first layer and the second layer there is a middle layer,which is patterned, printed, colored or otherwise treated so that it hasa pattern that is visible through the top layer and gives the coveringthe desired outer appearance.
 6. The covering material according toclaim 1, wherein the first layer contains a substance improving itsresistance to wear, the substance comprising quartz powder, feldsparpowder, finely-divided silica or aluminum oxide.
 7. The coveringmaterial according to claim 1, wherein in the ionomer of the first layerthe cation content is 0.04–2.5 millimols/gram of the polymer blend andthe alkali cation content is 0.4–1.7 millimols/gram of the polymerblend.
 8. The covering material according to claim 1, wherein theresistance of the first layer is about 1–100 Mohm.
 9. The coveringmaterial according to claim 1, wherein the resistance of the conductivelayer next to the first layer is approx. 0.01–0.02 Mohm.
 10. Thecovering material according to claim 1, wherein the first layer containsA) a polymer of an olefin and an unsaturated carboxylic acid, whereinthe quantity of acid groups is 0.5–15 molar %, and B) a block polyetherpolymer composed of a polyether block and a polyamide or polyesterblock, in which case the acid groups in polymer A and the polyetherblocks in polymer B have at least partly coordinated alkali ions, and analkali metal is present in an amount of about 0.02–3 millimols/gram ofthe polymer blend.
 11. The covering material according to claim 10,wherein block polyether B contains polypropylene oxide, polyethyleneoxide or a compound polymer of these, block polymer B having at leastpartly coordinated or complexed the said alkali metal.
 12. The coveringmaterial according to claim 10, wherein the olefin in polymer A isethylene or propylene.
 13. The covering material according to claim 10,wherein in polymer B the polyether block is polyethylene oxide having amolecular weight within the range 300–20,000.